1. Field of the Invention
This invention relates to a process for manufacturing quartz glass optical members with excellent stability to irradiation of excimer lasers, particularly, ultraviolet excimer lasers of KrF, ArF etc. More specifically, the invention concerns a process for manufacturing quartz glass optical members suitable for lenses, mirrors and prisms as such which constitute optical systems of laser processing apparatus or lithographic apparatus using the excimer lasers mentioned above as a source of light.
2. Description of the Prior Art
With the recent trend of increased density in large scale integrated circuits (LSI), a technique of drawing finer patterns with finer lines has been required in the photo-lithographic technique of drawing integrated circuit patterns on wafers. To meet this demand, attempts have been made to shorten the wavelength of the exposure light source. For example, the lithographic stepper light source is in a stage of transition from conventional G rays (436 nm) or I rays (365 nm) to excimer laser of KrF (248 nm) or excimer laser of ArF (193 nm). Lenses used for steppers are required to be very homogenous and permeable to ultraviolet rays.
In a shorter wavelength range than I rays (365 nm) mentioned above, sufficient light permeability can not be obtained with conventional multicomponent optical glasses. For this reason, it has been the practice to use quartz glass, particularly synthetic quartz glass (i.e., synthetic silica glass) with less impurity content in order to reduce ultraviolet ray absorption as much as possible.
To avoid contamination with metallic impurities which bring about ultraviolet ray absorption, the synthetic quartz glass is usually prepared from pure volatile silicon compounds synthesized chemically and purified by distillation. Such silicon compounds are silicon halides e.g. silicon tetrachloride (SiCl.sub.4) or alkoxy silanes e.g. tetraethoxy slane (Si(OC.sub.2 H.sub.5).sub.4) and tetramethoxy silane ((SiOCH.sub.3).sub.4) or alkylakoxy silanes e.g. methyltrimethoxy silane ((SiCH.sub.3 (OCH.sub.3).sub.3) or siloxanes e.g. hexamethyldisiloxane. In the preparation of the synthetic quartz glass, the vapor of a given compound such as those mentioned above is directly introduced into an oxyhydrogen flame for flame hydrolysis therein. As a result, fine glass particles are produced. These glass particles are grown by fusion deposition directly on a rotating heat-resistant core rod, thus obtaining transparent pure quartz glass.
In another process of obtaining quartz glass, the fine glass particles are not directly fusion deposited, but they are deposited on a heat-resistant core rod to form porous silica material, which is then made transparent by heating it in an electric furnace.
The synthetic quartz glass manufactured in the above ways is highly pure and results in good permeability down to a short wave length range of about 190 nm. Thus, it is frequently used as material for ultraviolet lasers, particularly as a light-permeable material for ecximer lasers of KrF or ArF.
However, although the method of improving the light permeability of ultraviolet lasers by improving the purity of the synthetic quartz glass is effective to a certain extent, sometimes the durability may be insufficient when exposed to long time irradiation of the excimer laser of KrF, ArF or the like. The reason is that the excimer laser beam is a pulse beam having a pulse duration of about 20 nsec (nanoseconds), that is, its energy per unit time is very high compared to ultraviolet rays emitted from the usual mercury lamp or others, and a very high load is applied to the glass.
To preclude the above drawback, the applicant has proposed a technique of increasing the ultraviolet laser beam resistance by doping the synthetic quartz glass with hydrogen, as disclosed in Japanese Patent Application No. 145226/1989 (U.S. Pat. No. 5,086,352). This technique is very effective. Actually, synthetic quartz glass containing 1.times.10.sup.17 molecules/cm.sup.3 or more of hydrogen meets requirements of a material for a KrF lithographic optical member. The technique thus has been carried out as industrially effective means as well.
The technique of hydrogen doping by heating the synthetic quartz glass in a normal pressure or pressurized hydrogen gas atmosphere is disclosed in the above application.
Such a hydrogen-doping technique is also disclosed in Japanese Patent Laid-Open Publication No. 201664/1989. Particularly this publication shows a technique to permit the doping with hydrogen by heat treatment in a hydrogen gas atmosphere at standard pressure at 800 to 1,000.degree. C.
The technique shown in the application was based upon the standpoint of hydrogen concentration and OH group concentration. However, as a result of extensive study conducted by the inventor on the behavior of paramagnetic defects that are generated by laser beam irradiation, the inventor found that paramagnetic defects are not always suppressed in the entire quartz glass so long as hydrogen is contained in a predetermined concentration, but there is a scatter of laser beam resistance depending on the state of the material. This scattered distribution of laser beam resistance directly leads to a scattered distribution of the service lives of the optical members. Therefore, this phenomenon constitutes a fatal disadvantage for industrial use of this kind.
Further, the paramagnetic defects generated by laser beam irradiation have an absorption peak at 215 nm. Therefore, although the problem is not so significant with the KrF laser (248 nm) which is spaced away from the absorption peak of the paramagnetic defects, it is a serious problem in the case of the ArF laser (193 nm) which is close to that absorption peak.
In the meantime, the excimer laser beam of ArF or KrF has very high energy and can directly break chemical bonds. Thus, a process for stripping cable wires or a process for piercing holes through a metallic or polyimide resin panel can be done more quickly without generating excessive heat using an excimer laser in comparison with a conventional YAG laser or a carbon dioxide gas laser. Since no excessive heat is evolved, it is possible to ensure accurate processing. Excimer lasers thus will be a promising field in industry.
However, the material applied to such optical systems is limited to synthetic quartz glass in order to be satisfactorily permeable to ultraviolet radiation. Even if the material is synthetic quartz glass, it is subject to optical damage because of the high energy of the irradiated light. Thus the problem of durability occurs frequently up to now.
In particular, the excmier laser beam energy that is required for laser abrasion processing is extremely intense since the necessary energy should be sufficient to decompose and vaporize polyimide or metals in the abrasion process. Optical members like those used in excimer laser lithography can not be applied in this technical field to obtain sufficient effects although they may be durable as laser lithographic lens material.